Traversing Space on a Bridge of Stars

A Bridge of Stars between the Magellanic Clouds

The Magellanic Clouds.

The Magellanic Clouds in the night sky. The Large and the Small Magellanic Clouds are visible. The Clouds are moving towards the bottom left corner. Credit: V. Belokurov and A. Mellinger

If you’ve ever been south of the equator, you’ve probably seen the Magellanic Clouds in the southern hemisphere sky. These two little galaxies  look like puffy clouds separated by a whole lot of space. It turns out that the light-years between them might not be so empty as astronomers once thought. Researchers at University of Cambridge in England have found what looks like a 43,000 light-year-long bridge of stars stretching from one galaxy to the other.  Their work, based on a huge census of stars that the Gaia satellite is doing, is giving a new look at what happens when dwarf galaxies interact. The result of its mission, when completed, will be a 3D map of our galaxy, and apparently of our neighboring satellite galaxies.

Using Old Stars to Trace a Bridge

bridge of stars

The Magellanic Clouds, their stellar halos and the RR Lyrae bridge. Pale white veils and the narrow bridge pf stars between the Clouds represent the distribtuion of the RR Lyrae stars detected with the data from the Gaia satellite. Credit: V. Belokurov, D. Erkal and A. Mellinger

The team of astronomers focused their attention on data about stars called RR Lyraes. These are pulsating variables that are quite old stars. They’ve been around for a long time — at least as long as the Magellanic Clouds have existed. So, their very existence tells us something about the history of these two nearby dwarf satellite galaxies. Theastronomers used the RR Lyraes to measure the extent of the Large Magellanic Cloud first. It turns out there’s a sort of fuzzy halo of these stars stretching away from the LMC that’s being stretched out into a evanescent bridge of stars.

The big question now is why this stream exists. Normally streams of stars aren’t stretching away from a galaxy unless there’s been something to tear them away. In this case, it’s likely that the tidal pull of the e Small Magellanic Cloud has steadily lured away stars from the LMC.  As it orbits, the LMC is leaving a tracer of its stars as it goes.  There could also be stars in the stream that are being attracted by the gravity of the Milky Way, too.

A Bridge of Stars During Interactions

Interactions between galaxies often warp and reshape the participants in the galactic dances. Such interactions are also an integral part of the galaxy assembly process: big galaxies get built from the collisions of smaller ones. We’ve seen streams of stars in other interacting galaxies, so this lovely bridge between the Magellanic Clouds fits right into the idea that cosmic dances can do more than warp galaxies. They can strip stars away, too.

This is a pretty cool story of galaxy evolution taking place in our own galactic back yard. If you want more information on the work the Cambridge astronomers are doing, check out their press release here.




Galaxy in the Shape of a Hummingbird

Could the Cosmos Get Any More Fascinating???

A long time ago, back when I was in graduate school, I used to collect those “newspapers” you see at the checkout counters in stores. You know the ones I mean—the ones with screaming headlines about alien babies and mutant animals and UFOs. Since I worked on a Hubble Space Telescope instrument team, I was particularly interested in seeing headlines about it. There were two stories that really made me laugh. The first was headlined “Astronomers Discover Galaxy in Shape of Fetus!!!!!!” and the other was “Hubble Space Telescope Takes Image of Heaven!!!!!”

This interacting galaxy duo is collectively called Arp 142. The pair contains the disturbed, star-forming spiral galaxy NGC 2936, along with its elliptical companion, NGC 2937 at lower left. Courtesy NASA/ESA Hubble Space Telescope/Hubble Heritage Team (STScI/AURA).

Of course, the image of heaven was an HST image of the Orion Nebula, which is a research heaven for people who are studying starbirth regions. But, there was no galaxy fetus out there. The editors of that story had found a press release about how astronomers had detected a faint rhythmic radio signal from a region in a distant galaxy, probably from a supernova or something. Somehow that got transmogrified into a baby galaxy headlined on a pulp rag.

There ARE lots of interestingly shaped galaxies out there, and this week Hubble Space Telescope astronomers released an image of and story about a distant galaxy that looks similar to a hovering hummingbird, sitting close to another elliptical galaxy. It’s called Arp 142, and it’s a snapshot of galaxies interacting. The whole dance of these two galaxies takes millions and millions of years, so we only get to see freeze-frame images like this one.

So, what’s going on here?  Let’s deconstruct the scene.  The “hummingbird” shape galaxy was a spiral galaxy before it began interacting with the elliptical just below it. The gravitational pull between the two galaxies warps the spiral and that affects the orbits of its stars and nebulae. Essentially, it warps the spiral, resulting in the shape you see here.  You can also see interstellar gas being pulled out of the spiral almost like a giant string of taffy.

The whole process compresses the gas and dust in the galaxy, which triggers star formation. You can see blobs of blue throughout the galaxy—those are starburst knots caused as the gravitational dance continues on.  The reddish dust  lanes used to be inside the galaxy. Now they’re being thrown out and compressed into the dark veins you see silhouetted against the bright starlight.

The companion elliptical, NGC 2937, is a puffball of stars with little gas or dust present. The stars contained within the galaxy are mostly old, as evidenced by their reddish color. There are no blue stars that would be evidence of recent star formation. While the orbits of this elliptical’s stars may be altered by the encounter, it’s not apparent that the gravitational pull by its neighboring galaxy is having much of an effect.

Above the pair, an unrelated, lone, bluish galaxy, inconsistently cataloged as UGC 5130, appears to be an elongated irregular or an edge-on spiral. Located 230 million light-years away, this galaxy is much closer to us than the colliding pair, and therefore is not interacting with them. It happens to lie along the same line of sight to foreground Milky Way stars caught in the image.

Now, I find this to be a MUCH more satisfying story than the kinds of things you read in the supermarket rags. Nothing their editors can dream up is anywhere NEAR as fascinating as what the actual cosmos is revealing to us through the watchful eye of the Hubble Space Telescope!